Normally, an image scanning device represented by a color image scanner and the like has light sources (e.g., LEDs) with emission wavelength characteristics of red (R), green (G), and blue (B), and scans information from a document using a common monochrome line image sensor while switching the ON/OFF states of them, and obtains two-dimensional image information while moving the monochrome line image sensor or document in a direction perpendicular to the arrangement direction of detection elements of the monochrome line image sensor (normally called a sub-scan direction) (e.g., Japanese Patent Laid-Open No. 2003-315931).
The luminous spectrum characteristics of the LEDs of respective colors as R, G, and B light sources used in the image scanning device are approximately as shown in FIG. 3.
On the other hand, the luminousity characteristics of human eyes have spectral sensitivity characteristics different from the emission wavelength characteristics of the LEDs, as shown in the CIE-RGB calorimetric system color matching functions shown in FIG. 4.
In order to compensate for these differences, the scanned image data undergoes color correction processes to improve color reproducibility of the scanned image. However, high color reproducibility has not been obtained yet.
Especially, an image scanning device using only R, G, and B light-emitting members cannot express a negative stimulus value of a red component which appears near a wavelength of 500 nm in the CIE-RGB colorimetric system color matching functions. Hence, the color reproducibility of an emerald system is prone to be poor.
In order to express a color that cannot be expressed by the scanning means using only R, G, and B primary colors, a method of extracting a color different from R, G, and B is known (Japanese Patent Laid-Open No. 2003-284084). This method is applied to a two-dimensional image sensor adopted in a digital camera, and detects one pixel by a plurality of types of extraction units which are limited to the wavelength ranges of R, G, and B and emerald color in place of switching light source colors so as to obtain color information from an object.
However, according to the technique of this reference, since data for one pixel is extracted by extraction units of independent colors, the light-receiving area of each extraction unit becomes too small to obtain a sufficient light-receiving amount. This imposes an influence on the S/N ratio. In addition, higher cost is required to manufacture such image sensing element, and it is difficult to apply this method to the image scanning device.